Mechanisms of
Evolution
Macroevolution
Speciation
The punctuated equilibrium model has
stimulated research on the tempo of speciation
• Traditional evolutionary trees - diagram the descent of
species from ancestral forms as branches that gradually
diverge with each new species evolving continuously over
long spans of time.
– a. The theory behind such a tree is the extrapolation of
microevolutionary processes (allele frequency changes in the gene
pool) to the divergence of species.
– b. Big changes occur due to the accumulation of many small
changes.
• We rarely find gradual transitions in the fossil record, but
often observe species that appear as new forms suddenly
in the rock layers and subsequently they remain essentially
unchanged through several strata and then apparently
disappear.
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• The punctuated equilibrium model
– Contrasts with a model of gradual change
throughout a species’ existence
Figure 24.13
Time
(a) Gradualism model. Species (b) Punctuated equilibrium
descended from a common
model. A new species
ancestor gradually diverge
changes most as it buds
more and more in their
from a parent species and
morphology as they acquire
then changes little for the
unique adaptations.
rest of its existence.
• This theory was proposed by Niles Eldredge (AMNH)
and Stephen Jay Gould (Harvard) in 1972.
– It depicts species undergoing most of their morphological
modification as they first separate from the parent species then
showing little change as they produce additional species.
– In this theory gradual change is replaced with long periods of stasis
punctuated with episodes of speciation.
• The origin of new polyploid plants through genome changes is one
mechanism of sudden speciation.
• Allopatric speciation of a splinter population separated from its parent
population by geographical barriers may also be rapid.
– For a population facing new environmental conditions, genetic drift and
natural selection can cause significant change in only a few hundred or
thousand generations.
– A few thousand generations is considered rapid in reference to the
geologic time scale.
• On the geological time scale, "sudden"
can refer to thousands of years.
• Differing opinions about the rate of
speciation may be more a function of
time scale perspectives rather than
actual conceptual differences.
• However, there is clear disagreement
about how much a species tends to
change after its origin.
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The origin of higher taxa
begins with the origin of evolutionary novelty
• Evolutionary changes can be traced
through the fossil record. The following
concepts address the relevant processes
that can cause the novel features that
define taxonomic categories above the
level of species.
The origin of higher taxa
begins with the origin of evolutionary novelty
• Most evolutionary novelties are modifications
of older structures
– Higher taxa such as families and classes are defined
by evolutionary novelties. Example:
• a. Birds evolved from dinosaurs, and their wings are
homologous to the forelimbs of modern reptiles.
• b. Birds are adapted to flight, yet their ancestors were
earthbound. Birds are at least partially defined by the
evolutionary novelties they possess for flight.
How New Designs Evolve
• One mechanism is the gradual
refinement of existing structures for
new functions.
– Exaptation - a structure that evolves in one
context and becomes co-opted for another
function.
– Exaptation provides an explanation for how
novel designs can arise gradually in
intermediate stages, each having some
function in the organism.
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How New Designs Evolve
• Natural selection cannot anticipate the future, but
it can improve an existing structure in context of its
current utility.
– Example: The honeycombed bones and feathers of birds did not
evolve as adaptations for flight. They must have been beneficial
to the bipedal reptilian ancestors of birds (reduction of weight,
gathering food, courtship), and later through modification, became
functional for flying.
• The evolution of novelties by the remodeling of
old structures for use in new functions reflects
the Darwinian idea of large changes resulting
from natural selection through an
accumulation of many small changes.
• Some complex
structures, such as
the eye
– Have had similar
functions during all
stages of their
evolution
• Read about the
evolution of the
vertebrate eye and the
cephalopod eye on
page 483.
Pigmented cells
(photoreceptors)
Nerve fibers
Nerve fibers
(a) Patch of pigmented cells.
(b) Eyecup. The slit shell
The limpet Patella has a simple
mollusc Pleurotomaria
patch of photoreceptors.
has an eyecup. Cornea
Fluid-filled cavity Cellular
fluid
Epithelium (lens)
Optic
Pigmented
nerve
layer (retina)
Optic nerve
(d) Eye with primitive lens. The
(c) Pinhole camera-type eye.
The Nautilus eye functions Cornea
marine snail Murex has
like a pinhole camera
a primitive lens consisting of a mass of
(an early type of camera
crystal-like cells. The cornea is a
lacking a lens).
transparent region of epithelium
(outer skin) that protects the eye
Lens
and helps focus light.
Optic nerve
Figure 24.14 A–E
Pigmented
cells
Epithelium
Retina
(e) Complex camera-type eye. The squid Loligo has a complex
eye whose features (cornea, lens, and retina), though similar to
those of vertebrate eyes, evolved independently.
Evolution of the Genes
That Control Development
• Genes that program development
– Control the rate, timing, and spatial pattern of
changes in an organism’s form as it develops
into an adult
– Can play a major role in the origin of
evolutionary novelties.
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• The evolution of complex structures
(e.g., wings) requires such large
modifications that changes at many
gene loci are probably involved.
• But, sometimes just a few changes in
the genome can cause major changes
in morphology. In these cases, slight
genetic changes can become magnified
into major morphological differences.
• Regulatory genes - In animals, a system of
regulatory genes coordinates activities of structural
genes to guide the rate and pattern of development
from zygote to adult.
• A slight alteration of development becomes
compounded in its effect on the allometric growth
(differences in relative rates of growth of various
parts of the body) that helps to shape an
organism.
– A slight change in these relative rates of growth will
result in a substantial change in the adult.
– Altering the parameters of allometric growth is one way
small genetic differences can have major morphological
impact.
Changes in Rate and Timing
• Heterochrony
– Is an evolutionary change in the rate or timing of
developmental events
– Can have a significant impact on body shape
• A change in allometric growth is a type of
heterochrony
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Allometric growth
Is the proportioning that helps give a body
its specific form
(a) Differential growth rates in a human. The arms and legs
lengthen more during growth than the head and trunk, as
can be seen in this conceptualization of an individual at
different ages all rescaled to the same height.
Newborn
Figure 24.15 A
2
5
15
Age (years)
Adult
• Different allometric patterns
– Contribute to the contrasting shapes of
human and chimpanzee skulls
(b) Comparison of chimpanzee and human skull
Chimpanzee fetus
growth. The fetal skulls of humans and chimpanzees
are similar in shape. Allometric growth transforms the
rounded skull and vertical face of a newborn chimpanzee
into the elongated skull and sloping face characteristic of
adult apes. The same allometric pattern of growth occurs in
humans, but with a less accelerated elongation of the jaw
relative to the rest of the skull.
Figure 24.15 B
Human fetus
Chimpanzee adult
Human adult
• Heterochrony
– Has also played a part in the evolution of
salamander feet
(a) Ground-dwelling salamander. A longer time
peroid for foot growth results in longer digits and
less webbing.
(b) Tree-dwelling salamander. Foot growth ends
sooner. This evolutionary timing change accounts
for the shorter digits and more extensive webbing,
which help the salamander climb vertically on tree
branches.
Figure 24.16 A, B
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• Another example of heterochrony is
paedomorphosis, also called neoteny
• Paedomorphosis - Retention of ancestral
juvenile structures in a sexually mature adult
organism
• In paedomorphosis
– The rate of reproductive development
accelerates compared to somatic
development
– The sexually mature species may retain
body features that were juvenile structures in
an ancestral species
Figure 24.17
• Also important in evolution is the alteration of
spatial patterns in development.
• Homeosis - Alteration in the placement of
different body parts (for example, to the
arrangement of different kinds of appendages in
animals or the placement of flower parts on a
plant)
• Because a regulatory gene may influence
hundreds of structural genes, there is great
potential for evolutionary novelties that define
higher taxa to arise much faster than would
occur by the accumulation of changes in many
individual structural genes.
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Homeosis
• Substantial evolutionary change
– Can result from alterations in genes that
control the placement and organization of
body parts
• Much of the classification of the different
groups of Phylum Arthropoda is based on
the number and placement of various
types of appendages (antennae and legs).
• Homeotic genes
– Determine such basic features as where a
pair of wings and a pair of legs will develop on
a bird or how a flower’s parts are arranged
• The products of one class of homeotic
genes called Hox genes
– Provide positional information in the
development of fins in fish and limbs in
tetrapods
Chicken leg bud
Region of
Hox gene
expression
Zebrafish fin bud
Figure 24.18
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• The evolution of vertebrates from
invertebrate animals
– Was associated with alterations in Hox
genes
1 Most invertebrates have one cluster of homeotic
genes (the Hox complex), shown here as colored
bands on a chromosome. Hox genes direct
development of major body parts.
Hypothetical vertebrate
ancestor (invertebrate)
with a single Hox cluster
First Hox
duplication
2 A mutation (duplication) of the single Hox complex
occurred about 520 million years ago and may
have provided genetic material associated with the
origin of the first vertebrates.
3 In an early vertebrate, the duplicate set of
genes took on entirely new roles, such as
directing the development of a backbone.
Hypothetical early
vertebrates (jawless)
with two Hox clusters
4 A second duplication of the Hox complex,
yielding the four clusters found in most present-day
vertebrates, occurred later, about 425 million years ago.
This duplication, probably the result of a polyploidy event,
allowed the development of even greater structural
complexity, such as jaws and limbs.
Second Hox
duplication
Figure 24.19
5 The vertebrate Hox complex contains duplicates of many of
the same genes as the single invertebrate cluster, in virtually
the same linear order on chromosomes, and they direct the
sequential development of the same body regions. Thus,
scientists infer that the four clusters of the vertebrate Hox
complex are homologous to the single cluster in invertebrates.
Vertebrates (with jaws)
with four Hox clusters
• Because a regulatory gene may influence
hundreds of structural genes, there is
great potential for evolutionary novelties
that define higher taxa to arise much
faster than would occur by the
accumulation of changes in many
individual structural genes.
Evolution Is Not Goal Oriented
• The fossil record
– Often shows apparent trends in evolution that
may arise because of adaptation to a
changing environment
Recent
(11,500 ya)
Equus
Pleistocene
(1.8 mya)
Hippidion and other genera
Nannippus
Pliohippus
Hipparion Neohipparion
Pliocene
(5.3 mya)
Sinohippus
Megahippus
Callippus
Archaeohippus
Miocene
(23 mya)
Merychippus
Hypohippus
Anchitherium
Parahippus
Miohippus
Oligocene
(33.9 mya)
Mesohippus
Paleotherium
Epihippus
Propalaeotherium
Eocene
(55.8 mya)
Pachynolophus
Orohippus
Key
Figure 24.20
Hyracotherium
Grazers
Browsers
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• Extracting a single evolutionary progression from
the fossil record is likely to be incomplete and can
be misleading.
• Example: By selecting certain species from available
fossils, it is possible to arrange a succession of
animals between Hyracotherium and modern horses
that shows a trend toward increased size, reduced
number of toes, and modification of teeth for grazing
(Figure 24.20, yellow line). Consideration of all known
fossil horses negates this trend, and reveals that the
line to modern horses is one of a series of species
lineages.
• Branching evolution (cladogenesis) can produce a trend even if some new
species counter the trend. Species that
endure the longest and generate the
most new species determine the
direction of major evolutionary trends.
This differential speciation is similar to
the idea of differential reproduction in
populations.
• Evolutionary trends more commonly
result from the gradual modification of
populations in response to
environmental change. See Figure 12
in Chapter 23.
• The appearance of an evolutionary
trend does not imply that there is some
intrinsic drive toward a particular
phenotype
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